1. Field of the Invention
The present invention relates to active thin film transistor circuit structure with current scaling function and, more particularly, to active thin film transistor structure applied for active light emitting device or the pixel circuit for an array of a display panel.
2. Description of Related Art
Organic light emitting diodes (OLEDs) attract people's attention recently since they are capable of illuminating without backlight modules to display images. In addition, OLEDs also have other advantages such as high contrast ratio, high brightness, wide viewing angle, high color saturation, and short response time. Moreover, OLEDs are light, thin, and low power consuming. Hence, OLEDs are possible to be widely used for displaying images.
The common transistor circuit for driving display devices can be a voltage-driven transistor circuit. However, this voltage-driven circuit and the method cannot compensate the characteristics variance caused by time-dependent decay of thin film transistor, the variation of the threshold voltage, and the variation of the field effect mobility. Therefore, the distribution of the brightness of the display devices is not uniform.
Another current driven circuit is shown in FIG. 1. The corresponding elements of the driven circuit are shown in table 1. The corresponding elements of the driven circuit are shown in table 1. The current driven circuit works by retrieving current from the external integrated circuits directly, and further homogenizes the distribution of brightness by compensating the variation of the threshold voltage of the transistor 105 and that of the mobility. Unfortunately, parasitic capacitors exist and generate as the data lines 109 intersects scanning lines 108. When the light-emitting devices 102 display in low gray levels, the driving current has to charge the parasitic capacitors and the storage capacitors first. Therefore, the response time of the light-emitting devices 102 is seriously delayed.
TABLE 1Function of the elementCharacteristicsThe required elementTransistor 104Switch1. being able toTransistor 106Switch   compensate theTransistor 107Switch   variation of theStorage capacitorStoring voltages for   transistor 105103turning on the transistor2. without the105 and for passing   function ofcurrent   reducing theAuxiliary element   currentTransistor 105Driving3. serious delayingLight-emitting 102Displaying   the response time   for low gray level
Another known current driven circuit by accompanying current mirror is shown in FIG. 2. According to the third illustration of the characteristic column of the table 2, it is known that the current scaling ratio needs to be increased if the improvement of the delaying of the response time is required. Furthermore, the current scaling ratio is proportional to the ratio of the geometrical size of the transistor 202 to that of the transistor 203. For example, when each pixel area of a display device is 125×125 μm2 pixel area and the width of the transistor 203 is 50 μm, the ratio of the geometrical size of the transistor 202 to that of the transistor 203 is limited under 2.5/1. In addition, the aperture ratio, i.e. the ratio of the area on the pixel without pixel circuit to the total area of the pixel, is reduced as the geometrical size of transistor 202 increases.
TABLE 2Function of the elementCharacteristicsThe required elementTransistor 202Switch1. to compensate theTransistor 204Switch   variation of theTransistor 207Switch   transistor 203Storage capacitor 206Storing voltages for2. being able to scaleturning on the transistor   the current203 and for passing3. the ratio of currentcurrent   scaling is constantAuxiliary element   and proportional toTransistor 203Driving   the ratio of theLight-emitting 205Displaying   width of the   transistor 202 to   that of the   transistor 203.
According to the description illustrated above, the display industry has a demand for an active current-scaling transistor circuit to improve the time delaying as low gray level is provided for displaying.